Familial influences on tests of mental and motor performance at ages four and seven were examined by comparing correlation coefficients among monozygotic twins, dizygotic twins, full siblings, half siblings, and cousins within race, sex, and social class groups. The correlations between scores of twin and sibling pairs on the Stanford-Binet (age 4) and Wechsler (age 7) intelligence tests suggested a greater genetic influence than was found for infant test scores.